1. Field of the Invention
The present invention relates to a method for forming a color image and, in particular, a method for forming a color image with proper optical density.
2. Description of the Related Art
Recently, the demand for high-quality output images of electrophotographic color image forming devices, such as color printers and copiers, has increased. In particular, density gradation and its stability are main factors of the human evaluation of image quality.
However, in such color image forming devices, optical density of the output image changes with variations of parts caused by the environment and aging effects. In particular, in electrophotographic color image forming devices, even a slight environmental variation may tend to cause a density variation and ruin the color balance. Therefore, a constant density-gradation characteristic should be maintained at all times. For this purpose, color image forming devices have a gradation correction function using several types of process conditions, such as an amount of exposure and development biases, and a look-up table (LUT) for each color toner depending on absolute humidity. The color image-forming device then selects optimal values for the process conditions and gradation corrections based on absolute humidity measured by a temperature and humidity sensor.
Additionally, color image forming devices have been disclosed in which a gray patch of black (K) and a process gray patch of a mix of cyan (C), magenta (M), and yellow (Y) are formed on a print medium as test patterns, and a sensor (herein after referred to “a color sensor”) detects the colors of the patches on the print medium by comparing the colors to each other after fusing, and then outputs a mixture ratio of CMY that makes the process gray patch an achromatic color.
In these color image forming devices, the detection result is fed back to an amount of exposure and process conditions of an image forming section as well as to a color matching table and a color separation table and a calibration table. The color-matching table converts RGB signals output from an image processing section to a color space of the color forming device while the color separation table converts the RGB signals to CMYK signals, and the calibration table corrects a density gradation characteristic. Accordingly, the color image forming devices can appropriately control the chromaticity of the output images on the print media (refer to, for example, Japanese Patent Laid-Open No. 2003-084532 or Japanese Patent Laid-Open No. 2003-107835).
The output image of these color image forming devices may be detected by an external image reader, a color meter, or a photographic densitometer and the chromaticity may be controlled in the same manner as described above; however, use of a color sensor to detect the output image is superior in that all the control is performed in a printer. The color sensor employs three or more light sources, for example, light-emitting elements having different spectrums, such as red (R), green (G), and blue (B), or a light source which emits white (W) light and three or more filters having different transmittance spectra of, for example, red (R), green (G), and blue (B) disposed on a light-sensitive element. Thus, three or more types of output, such as R, G, and B outputs, can be obtained.
In these cases, chromaticity of a patch, which is a test pattern, depends on chromaticity of the print medium. Accordingly, although patches are formed on print media based on the same process conditions and gradation correction, two chromaticity values are different if the print media are different. In particular, since it readily transmits light from the background of the print medium, a low-density patch is significantly affected by the chromaticity of the print medium. A color image forming device generally has a plurality of paper feeders (print media feeders) to support different types of print media, and can feed the print media by loading the different types of print media to the corresponding feeders.
However, in these known color image forming devices, since a print medium, on which the test pattern is formed, is fed from a specific feeder, the chromaticity is adjusted for a different print medium from that used for subsequent image forming when other media feeders with different print media are employed. This prevents proper color correction. To overcome this problem and obtain proper color balance adjustment, users must set the chromaticity of the print media in the color image forming devices by themselves.